Cavity spot gaining in carrots

The California carrot industry needs a new weapon to control increasing, tougher strains of cavity spot disease, according to a University of California, Davis, plant pathologist.

Mike Davis recently reported on his 2005 research findings during the California Fresh Carrot Advisory Board's research symposium in Bakersfield.

He found additional samples of the fungal disease, caused by Pythium species, having resistance to mefenoxam, the active ingredient in Ridomil Gold fungicide, in Kern County. Ridomil Gold is the only chemical control for the pathogen in carrots.

Davis said large portions of a few carrot fields, if not entire fields, were recently abandoned because of the disease. “Every year we are discovering more-resistant strains, and when I say more-resistant that means at least 50-fold more insensitive (to Ridomil Gold) than the rest of the Pythium population. We need other materials or strategies to control cavity spot.”

Cavity spot is shown as horizontal, depressed lesions on mature carrot roots, making them unmarketable. The fungus prefers cool soil temperatures and thrives at 58 degrees.

Precautions with carrots are avoiding excessive irrigation and harvesting of carrots soon after they mature, since older roots are more susceptible to the disease. It goes to all varieties.

Two species, P. sulcatum and P. violae, also attack alfalfa roots and may appear without symptoms in celery, blackeye beans, wheat, cucumber, and beets. Nonhost crops include cotton, tomato, watermelon, corn, and potato.

Three-year rotation

Cultural control for Pythium in carrots is three-year rotations out of that crop or alfalfa.

Davis also commented on his investigations in response to reports by carrot growers that metam sodium fumigations were failing to control nematodes and soilborne diseases, including cavity spot.

The project dealt with soil samples from three fields with long histories with metam sodium use and three with no history of its use.

He said research in Australia showed that continual use of the chemical favors soil microorganisms that speed its breakdown.

Metam sodium, he explained, is stable until it contacts moist soil. At that point it degrades into methylisothiocyanates, the same toxic compounds released by a decomposing mustard cover crop.

Counter to the Australian findings, Davis said lab analyses of soil samples done by USDA in Riverside indicated no evidence of accelerated breakdown of metam sodium in fields having a long history of applications of the chemical.

Instead, he added, he found the rate of breakdown and efficacy are governed by the by the organic or moisture content or the tilth of the soil concerned.

Phosphorous uptake

In another project for the board, Davis has been tracing the relationship of mycorrhizal fungi in the uptake of phosphorous by carrots.

Structures of two species of these Glomus fungi work as extensions of up to 3 inches to the carrot root to help collect phosphorus, which is immobile in the soil, and other nutrients. The two species account for 90 percent of the mycorrhizal fungi populations.

In his studies of soils of conventional, organic, and wildland fields, repeated three times, Davis concluded that mycorrhizal fungi are not necessary if soil phosphorus is adequate. However, he said, too much phosphorus can harm the fungi.

“The colonization of mycorrhizal fungi decreases as you increase soil phosphorus, so you are getting no benefit from them when you have high levels of phosphorus.”

On the other hand, Davis said, healthy populations of the fungi could probably replace certain amounts of phosphorus fertilizers applied to carrot fields.

Spray Safe

Fumigant or fungicide treatments applied for soil pests, he added, are lethal to mycorrhizal fungi.

The symposium also heard about the newly launched Spray Safe Campaign, a proactive, public relations approach to pesticide drift concerns, from Marilyn Dolan, executive director of the Alliance for Food and Farming, Watsonville.

Dolan's organization was formed in 1989 to address food safety and farming issues with the news media. It is a voluntary public relations operation for agriculture and is composed of 50 members, including Western Growers Association, the California Farm Bureau Federation, commodity groups, and individual growers and shippers.

Dolan said the Spray Safe Campaign is akin to a neighborhood watch program, with members keeping a close eye on each other's field to avoid spray accidents and prevent additional regulatory hardships that might come as a result of such mishaps.

“No one feels we need more regulations, but we do feel that people in the industry need to be reminded to be careful and that if incidents occur, there will be repercussions,” she said.

The alliance has been “packaging” the program with a logo, bumper stickers, and other promotional materials. A key element is a series of forums with farmers discussing concerns with regulatory agencies, backed up by educational sessions on how to improve existing spraying practices and policies. Close ties also will be maintained with legislators and the media.

A checklist has been prepared to remind growers and their employees of the importance of what's being applied, field re-entry restrictions, wind patterns, and close liaison with applicators.

Started year ago

Wes Selvidge of Buttonwillow was one of the growers involved with the Spray Safe Program from the start. He said the effort was originated by farmers about a year ago after burdensome regulations were proposed for use of metam sodium fumigant.

“We decided that new regulations from Sacramento wouldn't solve problems, only create new ones, so we needed to come up with common-sense solutions in the field.

“It's not rocket science but just knowing what's going on on your farm and those around you. It will keep a lot of the tools we need in our hands,” Selvidge said.